In recent years, from the viewpoint of reducing CO2 emissions, the need for improving the fuel economy has increased, and for machine structural parts that are used for automobiles, industrial machines, and the like, it has been desired to increase the strength of the parts for the purpose of decreasing the size of the parts.
Also, from the viewpoint of reducing the production cost, there has mainly been used a hot-forged part (hereinafter, a hot-forged part that is produced without being subjected to heat treatment of quenching and tempering is referred to as a “non-refined hot-forged part”), in which a steel bar produced by hot rolling (hereinafter, an as hot-rolled steel bar that is produced by hot rolling is referred to as a “rolled steel bar”) is subjected to a forming process by means of hot forging without subsequently being subjected to heat treatment of quenching and tempering, that is, “refining treatment,” so as to give a desired strength to the steel bar.
Many of the hot-forged parts are subjected to the forming process by mainly being rolled-down in the axial direction of the rolled steel bar, which is a starting material.
However, some of the hot-forged parts are subjected to the forming process by mainly being rolled-down in the direction perpendicular to the axis of rolled steel bar, that is, in the direction perpendicular to the rolling direction scarcely by being rolled-down in the axial direction of the rolled steel bar. For the hot-forged parts subjected to the forming process by being rolled-down in such a direction, the state of distribution of inclusions and/or precipitates formed in the hot rolling (that is, the state of distribution in the rolled steel bar of inclusions and/or precipitates elongated in the axial direction) remains even after the hot forging. Therefore, there is a tendency for the fatigue strength against the stress in the direction perpendicular to the axis of hot-forged part to decrease (hereinafter, the fatigue strength against the stress in the direction perpendicular to the axis of hot-forged part is referred to as the “transverse fatigue strength”).
As the tensile strength of hot-forged part is increased, the transverse fatigue strength can also be increased. However, the increase in tensile strength of the non-refined hot-forged part produced without being subjected to refining treatment leads to a decrease in tool service life in the cutting process carried out after hot forging. For this reason, there arise problems of increasing cutting costs and an increase in cutting time.
Therefore, it is not necessarily desirable that the transverse fatigue strength of hot-forged part be improved by increasing the tensile strength.
In such a situation, Patent Document 1 (JP8-92687A) and Patent Document 2 (JP6-287677A) disclose “High strength and high toughness non-refined steel for hot forging and its production method” and “High strength non-refined steel for hot forging”, respectively, as described below.
That is, Patent Document 1 (JP8-92687A) discloses a “high strength and high toughness non-refined steel for hot forging” configured such that in a steel containing, by mass percent, Si: 2% or less (excluding 0%), S: 0.10% or less (excluding 0%), N: 0.02% or less (excluding 0%), O: 0.010% or less (excluding 0%), and unavoidable impurities, the steel further contains, by mass percent, C: 0.10 to 0.6%, Mn: 0.3 to 2.5%, Cr: 0.05 to 2.5%, V: 0.03 to 0.5%, Al: 0.060% or less (excluding 0%), and Ti: 0.005 to 0.03%, and still further contains, by mass percent, as necessary one or more kinds selected from a group of Pb: 0.3% or less (excluding 0%), Ca: 0.01% or less (excluding 0%), Te: 0.3% or less (excluding 0%), Bi: 0.3% or less (excluding 0%), Zr: 0.1% or less (excluding 0%), Hf: 0.1% or less (excluding 0%), Y: 0.1% or less (excluding 0%), rare earth metals: 0.1% or less (excluding 0%), and Mg: 0.1% or less (excluding 0%), the balance being Fe and unavoidable impurities, wherein 1×102 to 1×106/mm2 of inclusions each having an average crystal grain size of 0.1 to 5 μm are contained, and the inclusions are Ti oxides/nitrides, MnS, and composite compounds consisting mainly of the Ti oxides/nitrides and MnS; and a production method therefor.
Patent Document 2 (JP6-287677A) discloses a high strength non-refined steel for hot forging containing, by mass percent, C: 0.25 to 0.50%, Si: 0.40 to 2.00%, Mn: 0.50 to 2.50%, Cr: 0.10 to 1.00%, S: 0.03 to 0.10%, V: 0.05 to 0.30%, and N: 0.0050 to 0.0200%, further containing one or two kinds of Al: 0.005 to 0.050% and Ti: 0.002 to 0.050%, and still further containing Ca: 0.0004 to 0.0050% as necessary, the balance being Fe and unavoidable impurities, wherein
the carbon equivalent Ceq. (%) expressed by the formula ofCeq. (%)=% C+(% Si)/20+(% Mn)/5+(% Cr)/9+1.54(% V)is 0.83 to 1.23%, and
the bainite transformation index Bt expressed by the formula ofBt=31.2−100(% C)−6.7(% Si)+9.0(% Mn)+4.9(% Cr)−81(% V)is 0 or less.